Remove WA for borders ref test

[bjwbell]

The borders_a.html ref test had a WA to avoid displaying a spurious
black pixel at (0,0). Details are at
#2879

The WA can be removed since the bug is no longer reproducible.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4124

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[jdm]

r+

[bors-servo]

saw approval from jdm
at bjwbell@0f40780

[bors-servo]

merging bjwbell/servo/remove-borders-test-wa = 0f40780 into auto

[bors-servo]

bjwbell/servo/remove-borders-test-wa = 0f40780 merged ok, testing candidate = e4d607e

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/870
success: http://build.servo.org/builders/linux2/builds/868
success: http://build.servo.org/builders/android/builds/904
success: http://build.servo.org/builders/mac1/builds/1085
success: http://build.servo.org/builders/mac2/builds/1085
success: http://build.servo.org/builders/gonk/builds/56

[bors-servo]

fast-forwarding master to auto = e4d607e